The new GLSL pass in their compiler is CSE, constant sub-expression elimination. The CSE pass ran at the GLSL IR level is still rather simple but it's leading to a reduction in the number of generated command instructions for shaders. Eric noted that for the Unigine Tropics test the frame-rate went up by 11% with this new pass due to its heavy GL Shading Language usage. However, no metrics were provided for other GLSL-using Linux games.
The nearly 600 line patch for Mesa can be found right now on the Mesa development list. Once this new shader compiler pass is in the Mesa Git tree it will be benchmarked at Phoronix.